Lath Structure Research Articles

Dramatically enhanced creep rupture performance and failure analysis of 9Cr–CrMoV dissimilar welded joint

The relationship between the microstructure evolution and the corresponding creep rupture performance are systematically investigated. Dramatically enhanced creep rupture performance is achieved for the 9Cr–CrMoV dissimilar welded joint by adjusting the heat treatment process (HTP) of CrMoV steels before welding. The crystallographic structure illustrates that the longer normalizing process could lead to the larger grain size, and the shorter tempering time devotes to the residual lath structure and finer second phase particles. The creep rupture test results show that the welded joint using CrMoV steel with original HTP ruptures in the soft zone with an obvious necking. After adjusting the HTP of CrMoV steel, the creep rupture life increases from 9634 h to 16,108 h accompanied with the rupture location transferred to the weld metal (WM), together with the fracture morphology from dimples to cleavage planes under the same creep rupture parameters. The fracture transition and enhancement of rupture life is resulted from the simultaneous strengthening effect of the lath structure and the precipitated fine particles in the matrix.

Cyclic plasticity and damage mechanisms of Ti-6Al-4V processed by electron beam melting

Cyclic deformation and damage mechanisms in electron-beam-melted Ti-6Al-4V are investigated. As-built samples exhibit a graded microstructure over the height of 120 mm, with samples from the top having larger α-laths and higher plastic strain. After HIPing, the α-lath width is greater, with reduced grain misorientation, and lower microstructural and property gradients. In both conditions, the observed cyclic softening is dominated by a monotonic reduction in the friction stress and an increase in grain misorientation, suggesting the lath structure progressively fragments into smaller grains. As-built samples show typically lower fatigue life due to crack initiation from gas pores and lack-of-fusion defects.

Impact of precipitated phases on the microstructure and mechanical properties of eutectic Sn58Bi alloy

A new type of low-melting-point heterogeneous alloy (LMH), based on Sn58Bi alloy (SBE), was prepared using Cu, Zn, Ag, and Sb microalloying and rapid solidification. The addition of Cu and Zn elements in equal mass ratios resulted in the Cu5Zn8 phase being dispersed in the Sn55Bi3Cu3Zn (LMH3) alloy. Subsequently, the additional Cu reacted with Sn to form a nano-sized η′-Cu6Sn5 phase at the interface of the Sn-Bi eutectic phase. Sn53Bi3Cu3Zn2Ag2Sb (LMH32) was prepared by adding appropriate amounts of Ag and Sb based on LMH3, leading to the precipitation of Ag3Sn and SbSn phases in the SBE matrix. The different precipitated phases in turn produced different types of fiber textures that existed in the matrix phase of SBE, LMH3, and LMH32. In addition, compared to LMH3, the presence of Ag3Sn and SbSn phases in LMH32 caused the formation of Bi twins identified via electron backscattered diffraction. The twin planes (101̅1) and (11̅02) had corresponding rotation axes and rotation angles of 93.8°[7̅431] and 90.2°[5̅415̅], respectively. Nanoindentation results indicated that LMH32 exhibited higher creep resistance than Sn55Bi3Cu3Zn (LMH3) and SBE. The precipitated phase significantly refines the grain size and eutectic lath structure of the matrix phase (Sn and Bi). During the deformation process, the fine lath structure and the interfacial η′-Cu6Sn5 nanophase can hinder the slippage of the eutectic interface. Moreover, the presence of the precipitated phase forms a large strain gradient in the alloy matrix, which produces a significant back-stress hardening effect and induces substantial dislocation nucleation. The formation of more dislocations made the creep mechanism of LMHs depend on dislocation climbing.

Microstructure characterization and mechanical properties of 2.25Cr-1Mo steel pipes using pulsed current GTAW

The 2.25Cr-1Mo pipe was designed to withstand high pressures and temperatures in a variety of industries, including steam boiler pipes,offshore oil drilling, power generation, and petrochemicals. 2.25Cr-1Mo is a good choice for fusion welding and forming. Pulsed current GTAW providesthe advantage of joining thin sections with low heat input, leading to less deformation and warpage than TIG welding. It also allows for better weld pool management, enhanced weld penetration, and improved weld quality with less distortion and warpage. It also allows for better weld pool control, enhanced weld penetration, and improved weld quality. The goal of this research is to use a design of experiments technique to optimize process parameters and describe the microstructure and mechanical properties of 2.25Cr-1Mo pipes welded with pulsed current GTAW (DOE). The Taguchi method is used to choose the L9 (34) orthogonal array (OA) for four factors (pulse current (PC), background current (BC), pulse frequency (PF), and pulse on time (PO)) at three different levels on 2.25Cr-1Mo steel pipe joints. The pulsed current GTAW method is used to obtain the optimal combination for increased weld strength. To find the best process parameters for 2.25Cr-1Mo steel pipes, researchers used an analysis of variance (ANOVA). 220 A Pulse current, 120 A background current, 8 Hz pulse frequency, and 60 %pulse on are shown to be the best parameters for 2.25Cr-1Mo steel. Microhardness, Tensile, and Impact tests are done and compared to the parent material's properties. Some bainitic packets at the weld zone have finer lath structures, whereas others have coarser lath structures. The fracture morphology shows tear ridge, cleavage facets, and ductile dimples where failure occurs at the weld zone and it indicates ductility failure.

Experimental And Numerical Modelling of Cyclic Softening and Damage Behaviors for a Turbine Rotor Material at Elevated Temperature

In order to better understand the physical process of deformation and cyclic softening a 12% Cr martensitic stainless steel FV566 has been cyclically tested at high temperature in strain control. Increase in temperature was found to increase the cyclic life, softening rate and viscous stress magnitude. An increase in the dwell time led to the acceleration of the material degradation. The microstructure changes and dominating deformation mechanisms were investigated by means of scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The results have revealed a gradual sub-grain coarsening, transformation of lath structure into fine equiaxed sub-grains, and misorientation angle development in blocks and packets until material failure. Further, a unified viscoplastic constitutive model coupled with a physically-based damage variable, is proposed to capture the cyclic mechanical behavior and microstructural evolution of the material at elevated temperature. The mechanical strength can be reduced by the decrease in the dislocation density, the coarsening of the martensitic lath and the loss of the martensitic structure under cyclic loading. The proposed physically-based damage variable is driven by the evolutions of dislocation density and martensitic lath width. The good comparisons with test results mean that the proposed model can reasonably model the cyclic elastic-viscoplastic constitutive behavior of the material at high temperature.

Effect of heat treatment on microstructures and properties of electron beam welding CLF-1 steel

Vacuum electron beam welding was adopted for the reduced activation ferritic martensitic steel (CLF-1 steel) with a thickness of 32 mm, and a well-formed butt joint without metallurgical defects was obtained under the optimum technological parameters. After welding, the specimens were subjected to high temperature tempering (710 °C/120 min/AC (air cooling), abbreviation HTT) and incomplete annealing (840 °C/120 min/AC, abbreviation IA) heat treatment, and the microstructure and mechanical properties of the as-welded state and two different heat treatment states were compared. The results showed that the as-welded weld metal (WM) was composed of coarse lath martensite, and the heat affected zone (HAZ) was composed of a large amount of massive martensite and dispersed fine MX and M23C6. Compared with the welded state, the martensite structure of HTT state WM and HAZ were refined, nano-scale M23C6 was precipitated along the boundary of tempered martensite in WM and nano-scale MX was precipitated in tempered martensite lath structure, both of which played a role of dispersion strengthening. Compared with the HTT state, the martensite structure of the WM and HAZ in the IA state became coarser, and both M23C6 and MX tend to grow together. The fracture position of the tensile specimens of as-welded state, HTT state and IA state welded joints were all located on the base metal (BM), and the tensile strength decreased successively, while the yield strength and elongation increased successively. The average impact absorbed energy of the BM, as-welded state, HTT state and IA state welds were 236 J, 42 J, 241 J and 51 J, respectively. The impact performance of HTT state welds surpassed the BM.

Synergy of strengthening and toughening of a Cu-rich precipitate-strengthened steel

A high strength steel with a combination of ∼930 MPa yield strength and excellent low temperature toughness with an upper shelf energy of above 200 J and ductile brittle transition temperature (DBTT) of lower than −90 °C is developed. The strengthening and toughening mechanisms are investigated systematically based on the detailed characterization on microstructures including the matrix and precipitates. The results indicate that the steel is composed of a fine lath martensite with rod-like Cu precipitates. The high strength is achieved by a combination of solid-solution strengthening, dislocation strengthening, grain boundary strengthening and precipitation strengthening of Cu-precipitates. The instrumented Charpy impact results further indicate that the crack propagation is the main factor affecting DBTT while the dislocation density has an obvious effect on both crack initiation and propagation. The fine lath structure of the low carbon martensite enhances the crack resistance and delays the rapid unstable crack propagation at low temperatures. Both the strengthening and toughening are thoroughly discussed in details.

Design, fabrication, and characterization of distinguished lightweight and sustainable building materials

As one of the eco-friendly building materials, autoclaved aerated concrete (AAC) has received considerable attention in industrial and academic research groups over the last few years. Herein, new AAC composites with different ratios of fine sand, lime, cement, and gypsum plaster were prepared in an autoclave at a pressure of 10 bar for 8 h. The effects of adding a polymelamine sulfonate (PMS) superplasticizer to the prepared AAC composites on workability were investigated according to their physical and mechanical properties. The binding phase of calcium silicate hydrate was fully characterized via Fourier transform infrared (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The effect of AAC slurry foaming agent and water to solid ratio (W/S) was reported in detail. Furthermore, we studied the effect of ultrafine sand (<63 μm) reinforcement on the properties of AAC products.Under the specified curing conditions, the results showed that a strength value of 3.8 MPa with a low bulk density of 0.60 g/cm3 (600 kg/m3) was obtained from an AAC composite with 65 wt% fine sand, 15 wt% lime, 15 wt% cement, and 5 wt% gypsum plasters, containing 0.12 wt% Al powder and 45% water to solid ratio (W/S) mixtures. The strength value of the AAC composite can increase to 4.8 MPa, maintaining a low bulk density of 0.60 g/cm3 when the AAC slurry composite included 1% PMS at a W/S of 56% and 0.08% Al powder. In particular, the production of ultra low-density building materials can be efficiently controlled by mixing superplasticized AAC slurries with 0.16 wt% Al powder (at W/S = 56%), reaching a low bulk density of 0.39 g/cm3 (390 kg/m3) with a reasonable strength of 1.70 MPa for AAC samples. Interestingly, 50 wt% of fine sand with a particle size of about 90 μm is sufficient for tobermorite formation from the sand-lime reaction at designated hydrothermal conditions. However, replacing the large particles with ultrafine sand (<63μ) reduced the AAC strength characteristics. Besides, the compressive strength was improved by forming long tobermorite needles, lath structures, or both. In contrast, the gel-like material (poorly crystallized tobermorite) provides the lowest strength properties for AAC (about 3 MPa).

Heat treatment for microstructure and mechanical properties improvement of powder plasma arc melted 17Cr-2Ni steel containing boron

Plasma arc melted 17Cr-2Ni steel containing boron was studied under conditions of as-deposited and heat-treated state in this paper. The deposits showed high hardness and strength. However, it contained carbides network and martensite lath which are known as a deleterious phase to both mechanical and corrosion properties of steels. Various double austenitizing treatments were performed to improve the mechanical properties of deposits. At first, to reduce the degradation caused by carbides network, the deposits were subjected to different austenitizing temperatures of 1000, 1050 and 1140 °C during the homogenizing treatment. Homogenizing at 1140 °C was selected due to significant dissolution of carbides network and intragranular large size carbides (>2 μm). Second, to reduce the influence of martensite lath on the mechanical properties, the samples were subjected to isothermal quenching (IQ) and tempering at 420 °C for 60 min following the double austenitizing treatment, which were referred to as IQ1140 and T1140, respectively. Different from the microstructure of tempered martensite lath structure of the T1140, the microstructure of pearlite with carbide particles size less than 500 nm was found in IQ1140, which was verified to improve the tensile strength and ductility of steels from 1486 MPa and 0.5% to 1708 MPa and 3.8%, respectively. In addition, MX-type nanoprecipitate were found in the ferrite matrix of heat-treated steels. The deposits heat-treated by IQ1140 process was proved to have comprehensive high hardness, high strength and a certain ductility, with an improvement of 11%, 15% and 660% to the ones of deposits, respectively. • Heat treatment of plasma arc melted 17Cr-2Ni steel containing boron was evaluated. • Carbides were dissolved mainly in the homogenization treatment stage. • MX-type nanoprecipitate existed in intragranular ferrite matrix after heat treatment. • Two sizes of intragranular carbides existed in the isothermal quenched samples. • Modification of martensite is main manner to improve its mechanical properties.

Simulation of crystal plasticity of P92 heat resistant steel considering martensitic lath structure

Based on the theory of crystal plasticity, coupled with dislocation and lath hardening models, this paper establishes a crystal plasticity finite element model describing the high temperature creep of P92 steel. Open source software was used to generate lath models with an average size of 350nm, 650nm and 950nm to explore the effect of lath coarsening on the high-temperature creep behavior of P92 steel. The results show that the roughening of the slats increases the rate of creep deformation, resulting in a decrease in the service life of the material. Observing the slat model, it can be seen that the roughening of the slats enlarges the numerical gradient of stress and strain, and aggravates the overall plastic strain of the model. The coarsening of the slats accelerates the movement of dislocations, causing the density of movable dislocations to increase, and at the same time the shear strain amplitude of the slip system increases, thereby reducing the hardening behavior of the material.

The dislocation-based fatigue deformation mechanism of a RAFM steel under multi-axial loadings

Reduced Activation Ferritic/Martensitic (RAFM) steel is a candidate blanket material for a fusion reactor, which should have outstanding performance under extreme environments containing complex thermal and stress damage during operation. To date, the properties under a single-axial loading type of tensile, creep and fatigue have been extensively reported, while the multi-axial loadings were rarely studied. Here, the multi-axial fatigue behaviors of a RAFM steel were investigated in an axial-torsional cyclic tester with controlled strain and different loading paths. The results show that the fatigue behavior of the RAFM is strongly determined by the shear-stress condition, where the lifetime under the pure torsional loading is only 16% of the uniaxial cyclic loading one. The fatigue life of the specimens under multi-axial loadings with a phase angle of 0° or 180° are comparable to that of uniaxial tension-compression one. However, it was decreased by ∼28% as the loading angle changed to 90° with a circular path. Besides, we found the fatigue softening under the proportional multi-axial loading while additional hardening in the non-proportionally multi-axial cyclic loading related to the dislocation activities during loadings. Under the proportional-multiaxial loading and uniaxial tension-compression loading and torsion loading, the high density of dislocation was rearranged and annihilated by the cyclic shear stress, while the tempered martensite lath structure was mainly retained. However, under the non-proportional multi-axial loading, dislocations transform the lath structure into a finer cell structure, improving the resistance upon fatigue deformation.

Influence of the Evolution of 9CrMoCoB Steel Precipitates on the Microstructure and Mechanical Properties during High-Temperature Aging

In this study, the short-term aging was carried out to reveal the evolution of precipitates and mechanical properties of heat resistant 9CrMoCoB steel during the early creep, replacing the conventional creeping. The tempered martensite lath structure (TMLS) and precipitates were observed in the as-aged 9CrMoCoB steel. TMLS in the matrix underwent a transition to the polygonal ferrite after aging only for 300 h. In comparison, the mean diameter of the precipitates increased from 183 to 267 nm after aging at 650 °C for 300 h. Also, the mean diameter of the precipitates increased from 183 to 302 nm at 700 °C. The room-temperature and high-temperature strength of 9CrMoCoB steel decreased after high-temperature aging, which may be mainly due to precipitates coarsening. Many M23C6 phases precipitate in the prior austenite grain boundary (PAGB) and lath boundary. After aging 100 h, TMLS transformed into polygonal ferrite, and the size of the precipitate at the subgrain boundary was about 100 nm, while after 300 h of high-temperature aging, large precipitates appear (400 nm) in the matrix. After 200 h of high-temperature aging, the obvious growth of precipitates on the PAGB and lath boundary weakens the pinning effect on the PAGB and martensite lath boundary and accelerates the transformation of microstructure and mechanical properties.

Presence of έ and ε crystal structures in rapidly solidified intermetallic compound Ni5Ge3

The congruently melting, single phase intermetallic Ni5Ge3 has been subject to rapid solidification via drop-tube processing wherein powders, with diameters between> 850 and 150 µm, are produced. At low cooling rates (> 850 µm diameter particles,< 700 K s−1) the dominant solidification morphology, revealed after etching, is that of partial plate and lath microstructure in an otherwise featureless matrix. At slightly high cooling rates (850–150 µm diameter particles, 700–7800 K s−1) complete plate & lath microstructure observed, again imbedded within a featureless matrix. Selected area diffraction analysis in the TEM reveals that partial plate & lath are the disordered form of έ-Ni5Ge3 in a matrix of the ordered (double-superlattice) form. Partial plate & lath has a very different EBSD to the plate & lath structure. Histogram of the correlated grain orientation angle distribution across grain orientation in partial plate & lath microstructure sample from the> 850 µm fraction showing random grain orientation. However, grain orientation in plate & lath structure from 300 to 212 µm fraction displaying the distribution predominance of low angle grain boundaries.

Damage behaviour in different micro-regions of P92 steel weldment under sequential low cycle fatigue and creep loading

Understanding the evolution of microstructure and micro-region properties of weldments during cyclic loading is a challenge for the reliability assessment of high-temperature components. This work is devoted to quantitatively evaluating the variation of micro-region properties and corresponding responsible microstructural features of P92 steel weldments under interrupted fatigue tests and subsequent creep fracture. To achieve this target, high-resolution characterization techniques, including electron backscatter diffraction (EBSD) and nanoindentation tester were used. The results revealed that the microstructures in the inter-critical heat affected zone (ICHAZ) and adjacent regions, fine-grained HAZ (FGHAZ) and parent metal (PM), show significant sensitivity to fatigue loading. Notably, the grain size in the ICHAZ reaches saturation after fatigue cycles of 10% fatigue lifetime, and the weld metal (WM) keeps unchanged throughout the fatigue loading. After subsequent creep fracture, only the WM presents an increasing trend in the average grain size. Nanoindentation tests uncover that the reductions of the elastic modulus and microhardness in each region also present three cyclic softening behaviour during the fatigue process due to the evolution of martensite lath structure. However, due to nucleation and growth of cavities accelerated by carbide precipitation, relatively low elastic modulus and microhardness were observed after subsequent creep loading.

Microstructure effects on the machinability behaviour of Ti6Al4V produced by Selective Laser Melting and Electron Beam Melting process

Additive Manufacturing (AM) is gaining widespread attention over the last decade in the areas of product development and prototyping, across aerospace and medical domains. Nevertheless, the built parts are near-net shaped that usually require a machining finish to improve the surface quality and operational life.The current study aims to document the machinability behaviour of the Ti6Al4V parts fabricated by the AM process, which is based on powder bed fusion, namely, Selective Laser Melting (SLM) and Electron Beam Melting (EBM). Further, machinability parameters are correlated with the microstructure and mechanical properties of the material. Face milling operation is performed under finishing conditions to study the response on the cutting forces. The effect of Hot Isostatic Pressing (HIP) and the influence of the build direction on microstructure and machinability have been investigated for the two AM processes. The effect of crystallographic textures and microstructure morphology of α and reconstructed prior β phases on the mechanical properties was studied using shear tests. It was observed that the SLM process primarily produced homogenous and fine α lath structures with less grain boundary α (αGB) grains, leading to low shear resistance and hence generated low cutting forces. On the other hand, the EBM process produced microstructures with variable morphologies, including more grain boundary alpha (αGB) grains and widmenstatten structures which resulted in a higher shear resistance and forces. The results show a striking influence of the microstructure induced by the type of the AM process, build orientation, and the subsequent heat treatment on the shear resistance and their ability to be machined. Further, the crystallographic textures were not strong for both processes and seem not to impact the machining process. The results highlight a strong “process – property” link which demands optimization of the cutting process for the type of the AM process and the resulting microstructure.

Mechanical behavior and microstructural evolution of a bainite-based quenching-partitioning (BQ&P) steel under high strain rates

In this paper, the mechanical behavior of a bainite-based quenching-partitioning (BQ&P) steel under high strain rates in a range of 2200–3600/s was studied using Split Hopkinson Pressure Bar (SHPB) experimental technique. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were employed to investigate the microstructural change and fracture behavior. Results show that the stress-strain relation of the steel exhibits obvious strain rate dependence, i.e. strength increases as strain rate increases. After high strain rates deformation, twisting, directional arrangement or crystal rotation appeared for lath (Bainite&Martensite) structure, and a <100> fiber texture was formed, indicating that the slip mechanism operates under high strain rates. As the strain rate increases from 2200-3000 1/s to 3001-3600 1/s, the fracture frequency of samples significantly increases from 33.3% to 77.8%. The fracture of the BQ&P steel is characterized as 45° fracture mode with dimples near impact end and shear river pattern near the other end. Most of blocky retained austenite (RA) underwent martensitic phase transformation in the form of γ→α′ and/or γ→ε→α′ during high strain rates deformation, whereas only partial RA inside a filmy RA underwent martensitic phase transformation. The calculated adiabatic temperature rise, ~100 °C is considered to increase the stability of RA, which results in the occurrence of stress-induced γ→ε transformation under high strain rates. The increased frequency of fracture by increasing strain rates can be attributed into the delayed strain response and embrittlement caused by the martensitic transformation of RA under higher strain rates.

Strain and temperature contributions to structural evolution in a Re-containing 10% Cr-3% Co-3% W steel during creep

ABSTRACT A Re-containing 10% Cr-3% Co-3% W steel with low N and high B contents subjected to normalising at 1050°C and tempering at 770°C was crept at 650°C under the applied stresses ranging from 200 to 100 MPa with a step of 20 MPa. During creep both the temperature and the strain affect the tempered martensite lath structure that causes the thermal and strain-induced recoveries, respectively, and degradation of structure. Both these recoveries lead to premature failure. The microstructure in the gauge and grip portions of the crept samples were investigated separately to estimate the contributions of temperature and strain during creep to the evolution of tempered martensite lath structure of the Re-containing 10% Cr-3% Co-3% W steel. The W depletion from the solid solution was accelerated by creep strain during short-term creep. The lath width and particle sizes were stable during long-term ageing at temperature of 650°C in the grip portions of the crept samples. The width of martensitic laths was 350 nm after 13,600 h of long-term ageing. The creep strain facilitates the recovery of the structure causing the subgrain formation with a mean size of subgrains of 1.1 μm and particle coarsening in the gauge and neck sections. The precipitation of W-rich Laves phase particles was observed in both grip and gauge portions. The constant coarsening rate of M23C6 carbides was significantly lower during long-term ageing than that during creep, whereas the constant coarsening rates of Laves phase particles were comparable in both grip and gauge sections of the crept samples.

Microstructure and Mechanical Properties of Heat-Treated Ti-Al-Si Alloy Produced via Laser In Situ Alloying

Intermetallics based on gamma titanium aluminide (γ-TiAl) alloys are potential lightweight materials that can be used for high-temperature components. However, poor room temperature ductility hinders their full potential for producing components using conventional manufacturing routes. This work is focused on the study of the microstructure and mechanical properties of ternary Ti-Al-Si alloy fabricated through laser in situ alloying by utilizing laser engineered net shaping (LENS) technology with subsequent heat treatment. The produced samples were analyzed using a scanning electron microscope (SEM) equipped with an energy-dispersed spectroscope (EDS) to investigate the composition and microstructure. Phase identification was investigated with x-ray diffraction (XRD) and electron backscattered diffraction (EBSD). Vickers hardness testing machine was utilized to determine the microhardness values, while the yield strength and tensile strength were calculated from the hardness results. Based on the results obtained, the microstructure of the heat-treated and as-produced ternary Ti-Al-Si alloy comprised mainly of α2-Ti3Al, γ-TiAl and ζ-Ti5Si3 phases. The SEM and EBSD results confirmed the phases formed, especially the fine lath structures of α2 in the colonies of α2/γ lamellae. The quantity of ζ-Ti5Si3 present in the LENS produced ternary alloy contributed majorly to the reported microhardness values. The sample heat-treated at 1150 °C/15 min/air-cooled (AC)/950 °C /6 h/furnace cooled (FC) (sample E1) had the lowest microhardness value of 624 Hv (6118 MPa or 6.12 GPa), with a corresponding tensile strength of 2325 MPa and yield strength of 1022 MPa. This was ascribed to the occurrence of more colonies of α2/γ lamellar in comparison with other samples.

Investigation of microstructural attributes of steel surfaces through magneto-optical Kerr effect

Magneto-optical Kerr effect (MOKE) microscopy is a versatile technique to image magnetic domains and evaluate the behavior of magnetic materials. However, MOKE has been rarely applied to the research of low magnetic steels with complex microstructure including micron-sized features. This study reveals the potential of implementing MOKE microscopy for analyzing the microstructure and stress state of martensitic steels with high carbide density (example conducted on AISI M2 steel grade). For the first time, the resulting MOKE domain images are used to analyze the size and morphology of martensite lath structures without chemical etching of the material's surface. Furthermore, MOKE microscopy allowed the confirmation and volumetric fraction estimation of retained austenite, despite the small size and volumetric fraction of under 1%. The validity of the MOKE microscopy analysis is cross-checked with scanning electron microscopy of etched samples and X-ray diffraction. Finally, MOKE microscopy in combination with Vickers indentation is applied to determine the residual stress state of the selected steel, which was varied by processing the steel material with conventional heat treatment scheme and heat treatment including deep cryogenic treatment (DCT). The method is proven effective in indicating the residual stress qualitatively. The measurements proved, that the residual stresses are significantly reduced in DCT steel samples, as theoretically anticipated. The conducted pioneering work provides important research towards the application of MOKE microscopy for the research of steel microstructure, properties and residual stresses as well as delivers a new perspective for determining the effectiveness of DCT on the steel properties.

Effect of tempering temperature on the microstructure and creep resistance of a 10%Cr martensitic steel

The tempered martensite lath structure of a 10% Cr martensitic steel after normalizing and tempering at 750 °C was studied. The effect of reducing the tempering temperature from 770 by 20 °C on the microstructural parameters in steel was shown. Tempering at a lower temperature of 750 °C provides thinner laths and finer M23C6 particles (55 nm) than tempering at 770 °C. Such structural changes do not affect the creep rupture time at 650 °C and applied stresses of 180 and 160 MPa, whereas the positive effect occurs at 140 MPa, the time to rupture increases by more than two times from 1426 to 3909 hours.